18F‐Fluorination of 2‐Methyl‐6‐nitrobenzenesulfonate Ester and its Application for the Synthesis of an 18F‐Labeled Amino Acid**

Abstract

AbstractIn the development of an efficient 18F‐labeling method for the synthesis of PET tracers, it is essential not only to improve the efficiency of [18F]fluorine incorporation into a carrier but also to minimize non‐radioactive side products from the precursor. Highly reactive sulfonate esters are promising precursors for 18F‐nucleophilic fluorination under mild conditions. However, the expected labeling efficiency from the precursor is often hampered by its competitive degradation due to coexisting bases during 18F‐fluorination. In this report, we designed a 2‐methyl‐6‐nitrobenzenesulfonate (2‐MeNs) ester as a precursor for 18F‐fluorination, in which the methyl group suppresses hydrolysis of the sulfonate ester via steric hindrance. An increase in labeling efficiency in the 18F‐labeling of a neopentyl labeling group was observed for the neopentyl 2‐MeNs ester compared with that of the 2‐nitrobenzenesulfonate ester. Ultimately, we achieved the automated synthesis of an 18F‐labeled amino acid using a neopentyl labeling group by using a 2‐MeNs ester as a precursor.